1. Field of the Invention
The invention relates to multilayer greaseproof packaging materials having a carrier layer which is made of paper/cardboard or other suitable substances on a polymer basis.
2. Description of the Related Art
It has been known for a long time to provide paper and cardboard containers with coatings having a barrier effect for flavors or humidity/liquids. DE 41 09 983 A1 describes a flexible packaging container comprising a composite made of a paper layer and a thermoplastic layer or film. The thermoplastic layer or film material consists of starch, a synthetic, non-polyolefinic polymer containing hydroxyl groups, e.g. an oxygenated polymer, and softeners of natural origin, e.g. polyalcohols derived from starch. Said material can be melted by supplying heat and therefore, it is extrudable. DE 41 37 802 A1 proposes to laminate a cardboard with a coated paper web to obtain a rottable, liquid-repellent product. The coating of the paper web shall be effected on a starch basis, e.g. on potato starch basis. DE 42 94 110 discloses a coating dispersion which is prepared from copolymers of oxidized starch and styrene, butadiene, acrylic acid or similar polymerizable molecules. Said dispersion reduces the gas and water permeability of cardboard or paper.
However, it is often necessary to provide packaging materials which have a high resistance to grease. Thus, food for animals, bread and confectionery, sweets and chocolate require a packaging having a particularly high resistance to grease which is for example indicated by the so-called KIT numbers exhibiting values between 8 and 12. High KIT numbers represent high resistances to grease, values as from 6 already represent a good resistance to grease.
Corresponding commercially offered paper/cardboard packaging has usually been subjected to a grease-repellent surface and/or mass treatment. At present, mainly fluoropolymers are used for said mass treatment or surface treatment, up to 5% by weight of coating material being applied onto the material. Grease resistances of >6 to 8 can only be obtained by combining layer and mass treatment, grease resistances with KIT numbers of >12 cannot be guaranteed with the present systems. For example, packing dry food for animals with a low grease content (<10%) requires a mass treatment of the backside, in case of higher grease contents, a barrier is realized by mass treatment in combination with a surface coating.
Waste paper, paperboard and cardboard packaging are regularly disposed via the waste paper circuit. Thus, via the pulping process, the halogen polymers used as grease barrier either arrive at the virgin paper product or in the waste water of the process.
Starch ethers are known as auxiliary agents and starting materials in the paper industry. The properties used are described in detail in the pertinent literature. They are used in surface coating and coating, respectively, and in pigmented paper coatings. In accordance with the BGVV (Bundesinstitut für gesundheitlichen Verbraucherschutz und Veterinärmedizin), paper, cardboard, and paperboard admitted for food contact may also contain starch ethers (e.g. hydroxyethyl ether and hydroxypropyl ether). Further, starch ethers are used as a component of adhesives because of their good film-forming property and their water bonding capacity. Respective literature is to be found for example in Ullmanns Enzyklopäidie der technischen Chemie; W. Baumann/B. Herberg: Papierchemikalien—Fakten zum Umweltschutz (Springer-Verlag); O. B: Wüirzburg: Modified Starches: Properties and Uses (CRC Press).
Further, it is known that starch ether derivatives can be processed to foils/films from an aqueous solution, particularly using casting technology.
When preparing the starch ethers in accordance with the so-called Slurry method, the aqueous starch suspension is derivatized under alkaline conditions at temperatures of up to 50° C. The degree of substitution (DS) is substantially around 0.2. The preferred derivatization at the C2 atom is characteristic for said methods. Another method which is substantially known from scientific examinations (autoclave method) is based on alkaline-activated starch and realizes more homogeneous derivatizations at lower TS (i.e. dry substance) concentrations, the degree of substitution (DS), however, being adjusted similarly. Proceeding in accordance with said strategy is described in DE 42 23 471 A1, the starch ethers obtained according to this document being intended to be used for the preparation of films, particularly for use as overhead, copying, and printing films or for the surface finishing of special papers, and as packaging material. Further, it is indicated in said publication that the ether derivative films mentioned therein can be used in combination with other materials.